loongarch missing tests/qtest/machine-none-test.c support

[Peter Maydell]

The tests/qtest/machine-none-test has a cpus_map[] array which needs
to be updated to list a CPU that will work with the 'none' machine type
for the longarch64 target. This results in a warning message during
'make check':

$ QTEST_QEMU_BINARY=qemu-system-loongarch64 ./tests/qtest/machine-none-test
# random seed: R02Sb57df3405339b14ef8e45e32fb787d3d
1..1
# Start of loongarch64 tests
# Start of machine tests
# Start of none tests
WARNING: cpu name for target 'loongarch64' isn't defined, add it to cpus_map
ok 1 /loongarch64/machine/none/cpu_option
# End of none tests
# End of machine tests
# End of loongarch64 tests

Could somebody update the test appropriately, please?

thanks
-- PMM